1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries, such as lithium secondary batteries.
2. Description of Related Art
Lithium-ion batteries are light in weight and capable of high power. For this reason, lithium-ion batteries in recent years have drawn attention as batteries for hybrid automobiles. A battery for hybrid automobiles is required to show relatively uniform power characteristics over a wide range of charge depth, in addition to high power characteristics. The purpose of this requirement is to reduce costs in the system by simplifying the control algorithm for battery input power.
Lithium-containing nickel-manganese composite oxides have drawn attention as a low-cost positive electrode material because they use nickel and manganese, which are rich in reserve, in comparison with lithium cobalt oxides, which have conventionally been used as a positive electrode active material. In particular, the lithium-containing nickel-manganese composite oxides are considered as good candidates for a positive electrode active material in the batteries for hybrid automobiles.
Nevertheless, a non-aqueous electrolyte secondary battery using a lithium-containing nickel-manganese oxide as its positive electrode active material has the problems of low discharge capacity and poor power characteristics due to its high resistance.
In view of the above-noted problems, Japanese Published Unexamined Patent Application No. 2003-92108 proposes the use of a lithium-manganese composite oxide having a spinel structure to improve low-temperature power characteristics of the battery. However, the use of the lithium-manganese composite oxide having a spinel structure, which has a discharge potential of about 4 V (vs. Li/Li+), leads to the problems of insufficient power characteristics and moreover insufficient battery capacity. In addition, much research has been conducted on lithium-containing nickel-manganese-cobalt oxides as well. The lithium-containing nickel-manganese-cobalt oxides, however, tend to cause a great increase in resistance at later stages of discharge and are incapable of achieving uniform power characteristics over a wide range of charge depth, which means that the lithium-containing nickel-manganese-cobalt oxides are unsuitable for the batteries for hybrid automobiles.
U.S. Published Patent Application No. 2003/0108793 describes that the use of a positive electrode material Li(LiNiMn)O2 in which lithium is arranged at transition metal sites improves discharge capacity considerably. Nevertheless, this technique has not yet been satisfactory in terms of uniform power characteristics.